The invention relates to a method for the production of precision castings by centrifugal casting apparatus for the purpose according to the preamble.
What is especially involved is the production of parts from materials containing titanium for internal combustion engines in molds divided by at least one plane of division into annular mold parts with a plurality of mold cavities extending at least substantially radially from a centrifugation axis, the molds and a casting system being housed in a closed chamber.
A method disclosed by EP 0 686 443 A1 deals primarily with the selection of special mold materials which have an influence on the casting and solidification of materials containing titanium, such as
The invention also extends to such materials, but is not limited thereto. Also involved are other materials such as highly heat resistant nickel aluminides, especially materials which are highly reactive at their casting temperature, including also the materials named in EP 0 686 443 A1.
Possibilities of application are found in the field of internal combustion engines, e.g., for oscillating parts such as valves, connecting rods and piston pins in which mass, noise and temperature are important. Applications, however, are also to be found in the field of rotating machines such as turbine wheels, turbine buckets, compressor wheels and parts thereof; that is to say, all mass products in which manufacturing costs, precision and adherence to all product parameters are of decisive importance, for reasons which are described in EP 0 6868443 A1. Another interesting possible application is in biomedical prostheses such as implants.
In the method disclosed by EP 0 686 443 A1, several rings of castings are made around a central sprue runner, and are combined to form a tree or a cluster of castings even between the rings by the material hardened in the sprue runner. Consequently stripping the casting is difficult and time-consuming, since the castings are to some extent enmeshed with the mold parts and anchored in the mold. To strip it the mold or the stack of plates of mold parts as a whole must be dismounted in the casting chamber, taken out of it and stripped in the open air.
Without a vacuum lock the interior of the casting chamber becomes contaminated by the ambient air and its content of water vapor, and with a vacuum lock the dismounting of the mold is extremely complicated. In either case, however, the mold parts become contaminated in the open air. But even if the mold for producing only a single ring were to consist of only two ring plates, dismounting them inside of the casting chamber would be difficult and the contamination problem still remains.
There is still another consideration: most of the above-described materials are hard and brittle at room temperature: at temperatures between about 200xc2x0 C. and 300xc2x0 C. they are solid but still ductile. During the above-described disassembly of stacks of molds with embedded casting trees the latter are cooled to room temperature, so that when they are stripped out fractures occur due to brittleness, resulting in rejects. Furthermore, before each new casting the molds must be reassembled by hand and heated from room temperature to 600xc2x0 C. to 800xc2x0 C., which is not only time-consuming but also a waste of energy.
The invention is therefore addressed to the problem of devising a method of the kind described above and an apparatus therefor, which will facilitate stripping the mold and permit a highly automated production of precision castings in a vacuum or under inert gas without damage to the castings and without excessive energy consumption.
The solution of the stated problem is achieved by the invention, stated above, by the features in the specific part of claim 1 and, in the case of the apparatus referred to above, by the features in the specific part.
A highly automated production of precision castings in a vacuum or under inert gas and without damage to the castings and without excessive energy consumption is made possible thereby. In particular the production of precision castings which are mass products for use as engine components is greatly facilitated and lower in cost.
In the invention, stripping the castings from the molds can be performed within the cooling curve at the thermally most favorable point in time at which the cast material is already sufficiently solid but still has sufficient ductility. The molds also do not have to be cooled to room temperature but only to the removal or stripping temperature of, for example, about 300xc2x0 C., and they can be heated from there back up to the casting temperature of about 600xc2x0 C. to 800xc2x0 C., but this is necessary ideally only at the inner margin of the mold. Thus the energy required for heating the molds and the period of time to the next casting are approximately halved. Even in regard to the energy consumption of the entire apparatus there is still an energy saving of 20 to 25%.
The core of the invention thus consists in the fact that the mold parts or mold halves are reliably pressed together parallel to their plane of division despite the high rotatory speed and are carried while rotating, but they can be drawn apart mechanically to remove the castings after they solidify, without any catching or sticking and without the need for cutting a central sprue away manually and at great effort inside or outside of the chamber.
As a result of additional embodiments of the method of the invention it is especially advantageous if, either individually or in combination:
the precision castings when cast are united at their radially inwardly pointing ends by a circumferential ring of the solidified metal,
the rotational guides of the casting mold parts are moved relative to one another between the closed position and the open position when the casting system for casting the melt while the mold is closed is brought into the plane of division and the casting is performed, if the mold is opened after the melt has solidified, and if then the precision castings joined together by the ring are removed from the plane of division inside of the chamber,
in the chamber a manipulator system with a clutching device is disposed, by means of which first, with the mold closed, the castings are picked up by their ring and fixed, if then the movable mold part is removed from the castings and from the stationary mold part, and if then the castings are drawn by the clutching device from the stationary mold part and brought into an intermediate position between the opened mold parts, from which position the castings are taken out into the exterior.
the mold parts are brought in a coaxial position into two sets of guiding wheels of which at least one guiding wheel is driven,
the mold parts are heated in a coaxial position to a casting temperature by a heating system brought concentrically into the plane of division, and/or if
the mold parts, to achieve a directional solidification from the outside in, are heated at such a rate that a temperature gradient of at least 40xc2x0 C., preferably of at least 200xc2x0 C., diminishing radially from the inside out, is established in the mold parts.
Pursuant to additional embodiments of the apparatus of the invention it is especially advantageous if, either individually or in combination:
the rotational guides are movable relative to one another between a closed position and an extracting position, if the casting system can be brought into the plane of division to cast the melt with the mold closed, and if a manipulator system is present for extracting within the chamber the precision castings joined to one another,
the castings are provided on their outer circumference with circumferential guiding means and are held positively in coaxial position on sets of guiding wheels of which at least one guiding wheel can be driven,
one of the mold parts is movable together with the corresponding set of guiding wheels relative to the other mold part and the other set of guiding wheels in the direction of the centrifugation axis,
the manipulator system has a radially acting clutching device with which the clustered precision castings can be removed from the mold parts,
the clutching device has radially movable plungers,
the plungers can be operated by a central shaft and bell cranks,
the manipulator system is provided on its outer circumference with a heat insulating body and a concentric heating body to heat the mold parts,
the chamber consists of two parts with a plane of division and if the one mold part is mounted in the one chamber part and the other mold part in the other chamber part, and/or if
the chamber part in which the manipulator system is mounted is connected to a magazine.
At the same time the materials and material combinations can also be taken from EP 0 686 433 A1.